Instream Flow Needs Analysis - Okanagan Basin Water Board

FINALOkanagan Basin Instream Flow Needs2.52.0Flow (cms)1.51.00.50.0Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecFigure 2.7. Natural mean daily flows (light blue) for Pennask Creek, with flow time series superimposed for eachyear on record. The dark blue lines show the minimum and maximum diversion thresholds ascalculated using the proposed guideline for fish-bearing streams. Flows occurring between these twothresholds are available for diversion/allocation (example figure taken from Hatfield et al. 2003,Appendix D).The BCIFN approach also includes a method that can be applied to fishless streams. For our project wedid not apply this to any node. Residual areas defined for the Okanagan that had no perennial flow wereconsidered fishless and were excluded from our IFN calculations.For this project, streamflow data were provided as mean weekly flows (instead of mean daily flows) overa shorter period of record (1996-2006) than the standard 20+ years recommended for the BCIFN method.As a result, the BCIFN method required adjustment to accommodate these differences in data frequencyand duration. Data were assessed over 4-week extents (referred to as periods in the analysis) instead ofmonthly extents for calculation of median/percentile flows. This data structure led to assessment of 13periods instead of 12 months in the calendar year. With only 11 years of data and using only weekly data,percentile flows for each 4-week period of the calendar year were determined from 44 data points (i.e.4 weekly flows per period multiplied by 11 years) instead of the 560–620+ data points (i.e. 28–31 dailyflows per month multiplied by 20+ years) than would have been standard for the BCIFN approach.Similarly, calculation of the 80th percentile flow for setting the maximum diversion rate utilized 572 datapoints (i.e. 52 weekly flows per year multiplied by 11 years) instead of 7300+ data points (i.e. 365 dailyflows per year multiplied by 20+ years of data) used typically for the BCIFN approach. The effects ofthese adjustments on determining minimum instream flow thresholds and maximum diversion rates arediscussed in section 2.5.1In addition to determining BCIFN minimum risk instream flow thresholds and BCIFN maximumdiversion rates, results of our analyses were used to calculate a recommended minimum watershedconservation flow for each week of the calendar year. The watershed conservation flow establishes aESSA Technologies Ltd. & 26Solander Ecological Research